As electrooptical devices comprising liquid crystals, electrooptical devices comprising nematic liquid crystals such as those of DSM, TN, G-H or STN types have been developed and put to practical use. These electrooptical devices comprising the nematic liquid crystals have a defect of a very slow response in the range from several milliseconds to several ten milliseconds and thus are limited in their applications. The slow response of devices utilizing a nematic liquid crystal is attributed to the fact that the torque for moving molecules is based on the anisotropy of dielectric constant and thus the power is not strong. Among these backgrounds, a ferroelectric liquid crystal which exhibits spontaneous polarization (Ps) and a strong torque based on Ps.times.E (E: applied electric field) and is capable of a high speed response in the range of several .mu. sec to several ten .mu. sec has been developed by Meyer et al. (Le Journal de Physique, 36, 1975, L-69). Furthermore, there is disclosed a ferroelectric liquid crystal in Japanese Patent Laid-Open Publication No. 307837/1988, there have already been proposed several high speed electrooptical devices comprising ferroelectric liquid crystals.
A typical example includes a device in which the helical structure is released by the force of wall faces and the two molecular alignments parallel to the wall faces is changed by the polarity of an applied electric field (see, for example, U.S. Pat. No. 4,367,924).
The aforementioned device is composed on the assumption of the presence of a compound which exhibits such an ideal bistable states as is shown by a field response wave pattern in FIG. 1. However, no compound which exhibits such an ideal bistable states as described above has been found, and bistable liquid crystal s synthesized hitherto show a field response wave pattern in FIG. 2 but not field response wave pattern in FIG. 1. It is the present state that if a device which exhibits a response wave pattern as shown in FIG. 2 is intended to be used for a switching circuit of light, such a pattern has a profile that transmittance varies gradually with the variation of an applied voltage from the minus side to the plus side and thus the object cannot be accomplished sufficiently with such a simple change of applied voltage as "on" and "off". Moreover, a bistable state liquid crystal having been synthesized is hard to form a monodomain structure as an ideal molecular alignment in the stage of an S*c phase at no electric field, and it causes disclination (defect) or twist which is the disturbance of the molecular alignment. It is thus difficult to realize the aforementioned ideal bistable alignment in a large area. Furthermore, it has a low threshold value (voltage at which the brightness varies at a predetermined extent), so that the dynamic drive of it may cause the lowering of contrast or the decrease of the range of viewing angle. The bistable state liquid crystal hitherto synthesized has no memory effect, since it cannot exhibit a hysteresis as shown in FIG. 1 but exhibits only a hysteresis as shown in FIG. 2. Thus, it is necessary to impress continuously a voltage at v.sub.3 in FIG. 2 or to apply a high frequency in order to maintain a stable response in the S*c phase in the liquid crystal, and it cannot avoid a large energy loss.
Eventually, it is the present state that many problems remain unsolved in conventional ferroelectric liquid crystal electrooptical devices notwithstanding the earnest desire of a high speed liquid crystal electrooptical device which takes advantage effectively of an applied field and a bond having a strong molecular alignment obtained in a ferroelectric liquid crystal.
Thus, there have been conducted researches on liquid crystal compounds exhibiting tristable states, characterized in that the liquid crystal compound realizes a stable molecular alignment having a distinct light-dark contrast depending on electric field applied, generates a distinct threshold property and a distinct hysteresis as shown in FIG. 3, realizes easily dynamic drive and is capable of a high speed response.
As a result, there have been proposed livid crystal compounds having tristable states as disclosed in Japanese Patent Laid-Open Publications Nos. 316367/1989, U.S. Pat. No. 5 171 471 and U.S. Pat. No. 4 973 738.
However, the liquid crystal compounds represented by the formulae: ##STR3## which are liquid crystal compounds exhibiting tristable states, have defects in their preparation processes that they are required for optical resolution and thus result in the expensive preparation costs because they are asymmetric compounds. As one of the methods for improving tile defects without diminishing the tristable states, there is a method for dilution and extending by adding other organic compounds. There is also another problem in this case that the tristable states are diminished by the addition of the other organic compounds in an amount of 20% or more, in some cases 10% or more.